The Effect of Shear Wave Attenuation on Acoustic Bottom Loss Resonance in Marine Sediments

Abstract

Acoustic propagation experiments in shallow water over thinly-sedimented bedrock sometimes show high propagation loss, with resonant-like structure, in the 10 to 100 Hz band. This can be interpreted with the aid of plane-wave reflection models that associate high bottom reflection loss at certain combinations of frequency and grazing angle with shear-wave resonances in the sediment layer. In this paper, the seabed is modelled as a thin layer of elastic-solid sediment overlying a hard rock substrate, with plane interfaces at layer boundaries. The bottom reflection loss resonances are explained by both full wave-theoretical reflection loss calculations and a simplified coherent ray reflection loss model. For the ray model, two simplifying assumptions are applied: (1) there is no conversion of energy between compressional waves and shear waves at the water-sediment boundary, and (2) sediment shear waves are attenuated so strongly that they traverse the sediment layer once up and down only. The paper investigates the effect of sediment shear-wave attenuation on bottom reflection loss. We find that the resonance frequencies are controlled by the shear speed and sediment layer thickness, and that the amount of bottom reflection loss is sensitive to the shear-wave attenuation.